Impact of C/N ratio and temperature on nitrogen removal from reverse osmosis concentrate using the sequencing batch reactor process

This study investigated the stability and effectiveness of biological nitrogen removal from high-strength reverse osmosis (RO) concentrate using a sequencing batch reactor (SBR) process. We examined the influence of varying temperatures (5–25°C) and carbon-to-nitrogen (C/N) ratios (2:1–5:1), utilizing methanol as an external carbon source. The results demonstrated consistently high total nitrogen removal efficiencies (91–97%) at temperatures between 15°C and 25°C, regardless of the C/N ratio. Nitrification exhibited robust performance, achieving approximately 97% efficiency across this temperature range, even at lower C/N ratios. Denitrification efficiency, however, showed a notable increase with higher C/N ratios, reaching 99.38% and 99.07% at C/N ratios of 4:1 and 5:1, respectively. At 5°C, significant inhibition of microbial activity was observed, causing nitrogen removal efficiency to drop below 40% across all C/N ratios, highlighting the importance of temperature control in practical applications. Chemical oxygen demand (COD) levels remained relatively stable (40–60 mg/L) within the 10°C to 25°C range, but an increase in CODcr release at 5°C suggested organic matter accumulation due to reduced biodegradation. In conclusion, the SBR process proved effective for nitrogen removal from RO concentrate under moderate temperature conditions (&ge10°C), though supplementary treatment may be necessary for complete organic matter removal, especially at low temperatures. These findings contribute valuable insights for optimizing biological treatment strategies for RO concentrate reuse or discharge.